1. Field of the Invention
The present invention relates to a photographic lens consisting of two lens groups each group consisting of a single lens element, and an economy camera using the photographic lens.
2. Background Arts
A wide variety of photographic cameras with different functions have been used for various purposes. Lens-fitted photo film units are well known as a cheap convenient camera that allows anyone to enjoy photography with ease. The lens-fitted photo film unit, hereinafter called simply as the film unit, has a unit body that contains a roll of unexposed photo filmstrip, and photographic mechanisms like a photographic lens and a shutter device are incorporated into the unit body. So the film unit permits photography from the moment it is purchased. For the development, it is not necessary to remove the exposed filmstrip from the unit body, but the film unit as the whole may be forwarded to a photofinisher.
In order to provide the film units at low prices, the manufacture cost is cut by making the construction of the film units as simple as possible. For instance, the photographic lens is fixedly held on a lens holder having a constant stop aperture size, and the focus of the photographic lens is adjusted such that photographs with average qualities may be obtained in most cases. A standard type film unit containing a conventional ISO-135 type photo film cartridge uses a photographic lens which is designed to have a focal length of about 32 mm, an f-number of about 9.5/f, and a half view angle of about 34.degree..
Recently, a new type photo film cartridge, called IX-240 type, has been brought into the market. The IX-240 type has a container with a spool that can contain the entire length of a photo filmstrip therein and advance the filmstrip to the outside in response to rotation of the spool. While the filmstrip of the ISO-135 type photo film cartridge has a width of 35 mm, the filmstrip of the IX-240 type photo film cartridge has a smaller width of 24 mm. Correspondingly, the frame size of the IX-240 type is smaller than that of the ISO-135 type. Therefore, a film unit containing the IX-240 type photo film cartridge have a photographic lens which is designed to have a focal length of about 24 mm, an f-number of about 9.5/f, and a half view angle of about 35.degree..
The photographic lenses used in the film units are usually plastic lenses formed from synthetic resins by injection molding, and are constituted of one or two lens elements. Recently, many film units adopt a two-group two-element construction photographic lens. As a popular two-group two-element photographic lens, there are a lens system consisting of two meniscus lenses with their concave surfaces facing each other, and a lens system consisting of a positive meniscus lens as the front lens element and a positive double-convex lens as the rear lens element. In either case, the front lens element disposed on the object side always has a smaller radius of curvature and a larger effective aperture than the rear lens element disposed on the image side.
Generally speaking, spherical lenses are more preferable than aspherical lenses, considering formability and optical accuracy. If, however, all lens surfaces of the two meniscus lens elements are spherical, the spherical aberration of the photographic lens increases so much that it is hard to correct or compensate. Besides, the f-number of the photographic lens is limited to at most 11/f, so that the image surface illuminance is very dark in this configuration.
Therefore, it is usual for the two-group two-element photographic lens to form the front surface of the front lens element alone or the front surface of the front lens element and the rear surface of the rear lens element to be aspherical. According to this configuration, the spherical aberration is well corrected, and the f-number of 9/f is achievable.
On the other hand, in the photographic lens having a double-convex lens as the front lens element, the spherical aberration is easy to correct, even if all lens surfaces are spherical. However, the curvature of the field becomes so bad that the image is damaged conspicuously on the fringe. Therefore, the front surface of the front lens element is formed to be aspherical so as not to worsen the curvature of the field.
Since the front lens must have a smaller radius of curvature and a larger effective aperture, even a slight variation in the forming process can damage optical performance of the lens. Because the aspherical surface is difficult to form with accuracy, it has been hard to maintain the optical performance of the above conventional photographic lens at a desired level.
When inspecting surface accuracy, an interferometer is usually used for spherical surfaces, and a surface roughness meter for aspherical surfaces. Depending upon the results of measurement, conditions of the forming machine are adjusted or corrected. As the inspection with the surface roughness meter takes more time and results less information in comparison with the inspection with the interferometer, it is difficult to precisely measure the surface roughness of the front lens that has a larger effective aperture. Therefore, it is difficult to maintain proper forming conditions.
There is another method of inspecting aspherical surfaces of lenses wherein a Null collector lens is combined with an aspherical lens to inspect, and an image formed through the Null collector lens and the aspherical lens is inspected. The Null collector lens is constituted of an aspherical lens and a glass spherical lens. For inspecting an aspherical lens having a short focal length and a large effective aperture like those used in the photographic lens, the Null collector lens is so difficult to design that this method is not practical.